1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, such as a copying machine or a printer.
2. Description of the Related Art
An image forming apparatus that includes an intermediate transfer member is conventionally known as an electrophotographic image forming apparatus. The conventional image forming apparatus includes a first voltage power source (i.e., a power source circuit) that can apply an electric voltage to a primary transfer member disposed in a confronting relationship with a photosensitive drum via the intermediate transfer member. The intermediate transfer member includes a primary transfer portion at which the intermediate transfer member can contact the photosensitive drum. An electric potential of the primary transfer portion is maintained at a predetermined level (which is referred to as a “primary transfer potential”). Then, the conventional image forming apparatus performs a primary transfer process for primarily transferring a toner image formed on a surface of the photosensitive drum (which serves as an image carrier) to the intermediate transfer member in a state where a predetermined potential difference is formed between the photosensitive drum and the intermediate transfer member.
The conventional image forming apparatus repetitively performs the above-mentioned primary transfer process for each of a plurality of colors to form a plurality of color toner images on the surface of the intermediate transfer member. Then, the conventional image forming apparatus performs a secondary transfer process for secondarily transferring the plurality of color toner images formed on the surface of the intermediate transfer member to a surface of a recording material (e.g., a paper) in a state where a second voltage power source applies a predetermined voltage to a secondary transfer member. The conventional image forming apparatus includes a fixing unit that subsequently fixes the toner images transferred on the recording material.
As discussed in Japanese Patent Application Laid-Open No. 2001-175092, an endless belt is conventionally used as an intermediate transfer member (which is hereinafter referred to as an “intermediate transfer belt”). A transfer power source (i.e., a power source circuit) dedicated to the primary transfer is connected to a stretch member that stretches an inner circumferential surface of the intermediate transfer belt or to the primary transfer member. The power source circuit supplies current that flows in the circumferential direction of the intermediate transfer belt to perform a primary transfer operation.
The intermediate transfer belt rotates and moves in a direction that corresponds to the above-mentioned circumferential direction of the intermediate transfer belt. According to the configuration discussed in Japanese Patent Application Laid-Open No. 2001-175092, the primary transfer potential is formed at each primary transfer portion in a state where a partial voltage is generated when the current supplied from the current supply member (i.e., the stretch member or the primary transfer member), to which the transfer power source is connected, flows in the circumferential direction of the intermediate transfer belt.
However, according to the configuration discussed in Japanese Patent Application Laid-Open No. 2001-175092 in which the primary transfer operation is performed while current flows in the circumferential direction of the intermediate transfer belt, the primary transfer potential at the primary transfer portion of each image forming station is greatly influenced by the resistance value of the intermediate transfer belt and the distance from the current supply member.
More specifically, the primary transfer potential becomes lower if an image forming station is positioned far from the current supply member. In other words, there is the possibility of causing a large difference in the primary transfer potential between an image forming station positioned near the current supply member and the image forming station positioned far from the current supply member. If the primary transfer potential cannot be appropriately maintained at each image forming station, transferring a required amount of toners to the intermediate transfer belt becomes difficult. The images fixed on a recording material may have a transfer defect (e.g., defect in density).